The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
In general, an automatic shifting power transmission includes a number of gear elements and torque-transmitting mechanisms that are selectively controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is commonly coupled to an engine of the vehicle through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gear set. Shifting from a currently established speed ratio to a numerically adjacent speed ratio involves, in most cases, disengaging a torque-transmitting mechanism (off-going torque-transmitting mechanism) associated with the current speed ratio and engaging a torque-transmitting mechanism (on-coming torque-transmitting mechanism) associated with the new speed ratio.
Since most shifts involve numerically adjacent speed ratios (i.e., sequential shifting), the fluid control hardware is designed to minimize the number of modulated pressure control valves, as disclosed for example, in the U.S. Pat. No. 5,601,506 to Long et al., issued on Feb. 11, 1997, and assigned to the assignee of the present invention. In Long et al., a set of relatively inexpensive on/off relay valves selectively couple the various transmission torque-transmitting mechanisms to two modulated valves, such that a certain combination of torque-transmitting mechanisms can only be coupled to a given modulated valve, and for any shift to a numerically adjacent speed ratio, one of the modulated valves is coupled to the on-coming torque-transmitting mechanism, and the other modulated valve is coupled to the off-going torque-transmitting mechanism. While such an arrangement can significantly simplify the control hardware and reduce manufacturing costs, it essentially rules out non-sequential downshifting. Thus, if the transmission is operating in third gear, for example, and the engine load abruptly increases to a level for which first gear would be appropriate, the controller must successively perform sequential shifts from third-to-second, and from second-to-first, instead of skip-shifting from third-to-first. Performing multiple sequential shifts takes the same amount of time as if the shifts are made separately
Accordingly, there is room in the art for an apparatus and method for performing timely non-sequential downshifts in a transmission where the control valve configuration is optimized for sequential shifting.